Conventional surface protective films for flexible wiring circuits are adhesive-bonded polyimide films, called coverlay films, that are punched out with a die conforming to the pattern, or are screen-printed films of UV or heat curable overcoating agents having flexibility. In particular, the latter type is more advantageous in workability. Known such curable overcoating agents include resin compositions based on epoxy resins, acrylic resins or composites thereof. These compositions are frequently based on resins modified by the introduction of butadiene structures, siloxane structures, polycarbonate diol structures or long-chain aliphatic structures. Such modification provides improved flexibility and suppresses warpage due to cure shrinkage while minimizing the reduction in heat resistance, chemical resistance and electrical insulating properties inherent to the surface protective films.
However, with recent weight reduction and miniaturization of electronic equipment, flexible boards are reduced in weight and thickness and are consequently more significantly susceptible to the flexibility and cure shrinkage of the overcoating resin compositions. As a result, the curable overcoating agents do not satisfy performance requirements in terms of flexibility and warpage due to cure shrinkage.
For example, JP-A-H11-61038 (Patent Literature 1) discloses a resin composition including a polybutadiene block isocyanate and a polyol. Cured products of the composition have good flexibility and shrinkage factor, but are insufficient in heat resistance.
JP-A-2004-137370 (Patent Literature 2) discloses a polyamideimide resin produced through reaction of a polycarbonate diol and a diisocyanate compound into a polyurethane having a diisocyanate at both ends, and reaction of the polyurethane with trimellitic acid. However, cured products of the resin have unsatisfactory long-term reliability of electrical properties.
JP-A-2004-182792 (Patent Literature 3) discloses a polyamideimide resin with an organosiloxane structure. However, cured products of the resin have bad adhesion to substrates. Further, the reference requires the use of special solvents such as N-methyl-2-pyrrolidone. Such solvents can dissolve emulsifying agents in the screen printing, often resulting in problems.
JP-A-2006-348278 (Patent Literature 4) discloses a carboxyl group-containing polyurethane that has a polyol unit selected from the group consisting of polybutadiene polyols, polyisoprene polyols and hydrogenated polybutadiene polyols. With regard to the circuit pattern formation in the COF (chip on film) packaging systems as an example, the subtractive methods are currently a common technique in the production of circuits in the COF packaging systems. The carboxyl group-containing polyurethanes disclosed in Patent Literature 4 show sufficient insulating properties when used as insulating films for the circuits fabricated by the subtractive methods.
However, the flexible circuit boards are expected to have smaller pitches between circuits (for example, 20 μm pitches or less) with the developments in the semi-additive process.
To cope with narrower pitches, the development of resins having better electrical insulating properties has been desired.
On the other hand, polycarbonate diols prepared from dimer diols as materials (namely, polycarbonate diols having structural units from dimer diols) are known in the art (for example, JP-A-H10-231360 (Patent Literature 5) and JP-A-H10-251398 (Patent Literature 6)).
Further, polycarbonate diols that are prepared from polyols having a C10-20 alicyclic structure as materials (namely, polycarbonate diols having structural units from polyols with a C10-20 alicyclic structure) have been disclosed in the art (for example, JP-A-2006-312729 (Patent Literature 7)).
However, Patent Literatures 5 to 7 do not describe polycarbonate diols that have structural units from dimer diols and structural units from polyols having a C10-20 alicyclic structure.
Meanwhile, carboxyl group-containing polyurethanes prepared from dimer diols as materials are known in the art (for example, JP-A-2000-7909 (Patent Literature 8) and JP-A-2007-100037 (Patent Literature 9)).
Further, polyurethanes that are prepared from polycarbonate diols having dimer diol-derived structural units have been disclosed in the art (for example, JP-A-H10-273514 (Patent Literature 10) and JP-A-H10-251369 (Patent Literature 11)).
However, Patent Literatures 8 to 11 do not describe carboxyl group-containing polyurethanes that are prepared from polycarbonate diols having structural units from dimer diols and structural units from polyols having a C10-20 alicyclic structure.